This invention relates to a blank for conversion into a perforated stencil by spark erosion and the method of using the blank.
Stencil blanks for use in a spark erosion, stencil cutting machine are well known and commercially available. Such blanks are known which comprise a stout backing sheet or substrate on which is adhered an electro-responsive layer or layers made up of an electrically conductive region adjacent the backing sheet and an electrically resistive region on the conductive region. The backing sheet prevents shrinkage of the electro-responsive layer(s) between manufacture and use of the blank. The blank is converted into a stencil in a stencil cutting machine in which a stylus applied potential causes a current to flow through the resistive region to the conductive region. This current produces localised perforation of the electro-responsive layer(s) to form a stencil. The backing sheet is then peeled from the stencil which is placed in a stencil copier. If attempts are made to copy large black or grey areas of an original, the perforation density (i.e. the ratio of the sum of the areas of the perforations in a given area of the stencil to that given area) on the corresponding part of the stencil will be high. Thus, under such circumstances, peeling of the backing sheet from the stencil is a delicate operation, in order to ensure that the stencil is not torn in areas of high perforation density. Another disadvantage of such known stencils is that when providing the electrically resistive region on the conductive region during manufacture of the blank, chemical or physical interaction between the two layers may take place such as to affect the required properties of these regions.